Free floating mooring links

ABSTRACT

For floating platform horizontal stability, mooring lines can be floated by external water neutral-density means to remove anchor line curvature. Thus mooring lines could be pulled straight under water to provide optimal horizontal resistance, thereby reducing need of excessive mooring line pretension that increases platform loads. More savings are realized by not having to design floating platforms with extra capacity to handle added pretension and weigh of mooring line. Free Floating Mooring Links capitalize on the concept of overall neutral density with water, but instead of using cables or chains with external floatation means to keep anchor lines from sinking in water, FFML utilizes the shape of anchor line itself to create inherent built-in floatation so as to achieve overall system neutral density to water. Each FFML section has a longitudinal float that provides floatation and a connector that provides chain-like flexibility and connectivity, and the longitudinal float is in essence equivalent to arranging conventional cable wires into the shape of a pipe or tube that could be enclosed for buoyancy.

BACKGROUND OF THE INVENTION

As oil and gas operations extend farther and farther out into deeper ocean areas, new technology has facilitated the petroleum industry's ability to manage production in more difficult environments. At depth of 10,000 ft for example, mooring lines for horizontal stability are greatly affected by catenary curvature flexibility of anchor cables or chains, and large horizontal platform movements result as mooring line curvature changes with varying ocean current forces on platform. Furthermore, existing anchor systems must dedicate a substantial portion of their strength just to lift mooring-line dead weigh against gravity, not to say the least about the additional capacity that must be built into the platform to handle this load. Needless to say, compliant towers are compliant only because the industry has no effective means to truly restrict compliant-tower horizontal movement.

BRIEF SUMMARY OF THE INVENTION

Free Floating Mooring Links (hereinafter FFML) capitalize on floating metal under water to achieve overall neutral density with water, and effect of gravity can be negated. Instead of employing external floatation means to lift anchor lines, FFML incorporates floatation directly into each mooring link section for simplicity of system design and for production costs management. Also, FFML connectors provide flexibility so the entire mooring system could bend and move as conventional chains and cables.

DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an enlarge view of FFML float and connector.

FIG. 2 shows a longitudinal float with connector.

DETAIL DESCRIPTION OF THE INVENTION

Gravitational effects on a conventional mooring chain or cable could be neutralized by addition of a plurality of water neutral-density means. The current invention achieve overall system density neutral to water with floatation built into each FFML section, thus in the process saving use of additional materials to create external floatation devices. In an idealized scenario, a water neutral-density anchor line could be made up of one long hollow tube, or a free floating tendon; however, an excessively long floating tube may be hard to handle, subject to bending or buckling failure, and exposed to system risk from a single leak that could flood the entire system.

For the present invention sealed FFML float 10 has density less than water to accommodate added weigh of FFML connector. An FFML connector comprises cap 20 with a rigidly attached link 30 which interlocks with another link 30 rigidly connected to cap 40. The two links 30 enable the connector to behave like conventional chain links. Thus cap 20 and cap 40 could move relative to each other while providing rigid connection like chain links. Cap 20 could be attached to float 10 by threads or welds, and the connection could be manufactured water tight in factory or steel mill.

An FFML section comprising a longitudinal float 70 and a connector 90 which could be delivered as one unit to offshore location for installation. End plug 80 could be attached in factory or steel mill to ensure longitudinal float 70 is sealed for buoyancy; however, end plug 80 is not necessary if field connections of FFML sections could provide water-tight seals between each connector 90 and the next longitudinal float 70.

It should be obvious to engineers knowledgeable of the art that it is a simple mathematical exercise to determine the required ratio between outer and inner diameters of the previously discussed long tube for neutral density to water. For the presented FFML sections, the wall thickness of the longitudinal floats would have to be thinner than that based on the ratio of the long-tube diameters alone since additional weigh of connectors must be uplifted. Of course, adjustments to wall thickness relative to diameter would depend on connector design and length of each longitudinal float. The precise calculations should be obvious to those knowledgeable of the art.

As an exercise in preliminary design, consider a platform at 10,000 ft of water depth anchored by FFML with longitudinal floats having cross sectional area of 50 square inches, perhaps 24-inch diameter pipe with ⅔-inch wall thickness. Based on 45 degrees anchor line orientation, total mooring line length would be 14,142 ft. An axial force of 1,000,000 lbs would stress the floats to 20,000 psi longitudinal stress and stretch the system to an overall elongation of about 9.4 ft. Of course, a precise analysis must account for connector design and flexibility, not to say the least for the trigonometric effect from the 45-degree angle. However, it should be obvious from the above approximation that any movement allowed by FFML would be small compared to current deep water flexibility with platforms swaying some 300 ft horizontally. 

1. A mooring system having multiple link sections, with each link section comprising a connector and a longitudinal float; with said connector having flexibility and connectivity of chain links, and with said longitudinal float having adequate buoyancy such that the longitudinal float and the connector together have about the same density as water. 